Magnetic resonance imaging (MRI) is a major imaging technique used in medicine. MRI is capable of generating detailed images of soft tissues such as the brain, muscles and kidneys. Specific properties of the various compounds found inside tissues, such as water and/or fat, are used to generate images. When subjected to a strong magnetic field, the vector sum of the nuclear magnetic moments of a large number of atoms possessing a nuclear spin angular momentum, such as hydrogen, which is abundant in water and fat, will produce a net magnetic moment in alignment with the externally applied field. The resultant net magnetic moment can furthermore precess with a well-defined frequency that is proportional to the applied magnetic field. After excitation by radio frequency pulses, the net magnetization will generate a signal that can be detected.
Various electromagnets are integral parts of an MRI system. For example, they allow spatial encoding of the detected signals for the formation of spatial images, and correction of any irregularities. Electromagnets perform this function by generating magnetic fields with predetermined shapes. For example, gradient coils are typically designed to generate magnetic fields that vary linearly with a constant tangent along the three perpendicular axis of the MRI systems' imaging volume.
Manufacturing electromagnets which can generate magnetic fields with the desired requirements such as desired magnetic field shapes can present challenges. Specifically, to function properly, electromagnets are typically produced to operate in accordance with additional requirements besides magnetic field shape. For example, it is desirable to produce gradient coils, which when energized have minimal net force and torque. This requirement is in addition to the linearity of the magnetic field produced.
Typically used methods of force and torque balancing for gradient coils, however, assume that the external magnetic field such as that produced by a main magnet of the MRI is a uniform magnetic field in space pointing in the axial (z) direction. When the external field is uniform, as long as current enters and exits the gradient coils at the same location, net forces on gradient coils (or other electromagnets) may not be produced. However, typically the external magnetic field to which gradient coils or other electromagnets are subjected is non-uniform in the region where a gradient coil or electromagnet is placed. For example, short superconducting magnets can have significant radial and axial non-uniformities in the magnetic field generated where the gradient coil is typically placed. Accordingly, a gradient coil may experience significant net forces and/or torque when placed in a non-uniform field, even if current enters and exits the gradient coils at the same location. Thus, improved electromagnet design, manufacturing and operating techniques are needed to allow the construction of electromagnets in accordance with specified requirements.